The ink jet recording system is a printing method of ejecting and splashing small ink droplets through a fine nozzle and attaching these ink droplets on the surface of a recording medium such as paper, thereby forming a letter or a figure.
Examples of the ink jet recording system which is put into practical use include a method of converting electric signals into mechanical signals using an electrostrictive element and intermittently ejecting an ink stored in a nozzle head part, thereby recording a letter or a symbol on the surface of a recording medium, and a method of rapidly heating a portion very close to the ejection part to generate bubbles in an ink stored in a nozzle head part and intermittently ejecting the ink using the cubic expansion due to the bubbles, thereby recording a letter or a symbol on the surface of a recording medium.
The ink for use in this ink jet recording is required to have various characteristics. For example, no blurring and good drying property in the printing on a paper as a recording medium, capability of uniform printing on the surface of various recording mediums, and no mixing of adjacent colors in the multicolor printing such as color printing, are required.
The ink generally used for ink jet recording is an ink obtained by dissolving a water-soluble dye of various types in an aqueous medium. Also, an ink obtained by dispersing a pigment in an aqueous medium is provided. The ink obtained by dispersing a pigment in an aqueous medium is characterized in that the water resistance and light fastness are excellent as compared with the ink using a water-soluble dye.
On the other hand, the pigment has a problem in the dispersion stability when used for an aqueous ink composition. In this case, a dispersant such as polymer dispersant or surfactant is generally used so as to stably disperse the pigment in an aqueous medium.
Furthermore, for the purpose or elevating ejection stability, dispersion stability, printing density and coloring property, an ink using a pigment subjected to a some surface-treatment is proposed. For example, a pigment particle or which surface is treated by oxidation is disclosed in JP-A-8-319444 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), an encapsulated pigment fine particle is disclosed in JP-B-7-94634 (the term “JP-B” as used herein means an “examined Japanese patent publication”) and JP-A-8-59715, and a pigment particle of which surface is graft-polymerized with a polymer is disclosed in JP-A-5-339516, JP-A-8-302227, JP-A-8-302228 and JP-A-8-81647.
In addition to these proposals, for elevating the fixing property of pigment on a recording medium, for example, an ink using a pigment on which a resin having a film forming property is coated at a room temperature by the phase inversion emulsification method is disclosed in JP-A-8-218015, an ink using a pigment on which an anionic group-containing organic polymer compound is coated by the acid dipping method is disclosed in JP-A-9-31360, and an ink using a polymer emulsion obtained by impregnating a coloring material into a polymer fine particle according to the phase inversion emulsification method is disclosed in JP-A-9-286936.
In many cases, the printing quality of conventional inks using a pigment is ensured mainly by suppressing the permeability and thereby allowing a pigment as a coloring material to stay in the vicinity of the paper surface. However, such an ink has a problem in that the blurring greatly differs depending on the kind of paper and the adaptability to all kinds of paper is low. Furthermore, drying of the printed portion takes a time and particularly in the multicolor printing such as color printing, mixing of adjacent colors disadvantageously occurs.
With respect to the means for solving these problems, addition of a penetrant is proposed so as to improve the permeability of ink into paper. For example, a case of using a triethylene glycol monomethyl ether is disclosed in JP-A-56-147861, a case of using ethers such as ethylene glycol, diethylene glycol or triethylene glycol is disclosed in JP-A-9-111165, addition of a diethylene glycol monobutyl ether is disclosed in U.S. Pat. No. 5,156,675, addition of Surfynol 465 (produced by Nisshin Chemistry Co. Ltd.) as a acetylene glycol-type surfactant is disclosed in U.S. Pat. No. 5,183,502, a combination use of diethylene glycol monobutyl ether and Surfynol 465 is disclosed in U.S. Pat. No. 5,196,056, and use of ethers of diethylene glycol in ink is disclosed in U.S. Pat. No. 2,083,372.
The ink using a pigment has been heretofore produced by dispersing a pigment in an aqueous medium using a surfactant or a dispersant such as polymer dispersant, however, in this case, it is difficult to elevate the permeability of ink by using the above-described penetrant while ensuring the dispersion stability of the pigment.
In the case of dispersing a pigment using a surfactant or a dispersant such as polymer dispersant, the dispersant is merely adsorbed to the pigment surface and therefore, when the above-described penetrant is used to improve the permeability, the dispersant adsorbed to the pigment is eliminated due to the storage shearing force upon ejection of ink through a narrow nozzle, as a result, the dispersibility is deteriorated and sometimes, the ejection becomes unstable. Furthermore, when such ink is stored for a long time of period, the pigment is liable to suffer from unstable dispersability in some cases. For solving this problem, a method of increasing the amount of dispersant added may be thought out, however, in this case, the dispersant is highly probably not adsorbed to the pigment surface but dissolved in ink and this readily causes occurrence of blurring at the printing on plain paper or recycled paper or wetting of ink to the periphery of a nozzle, giving rise to factors of unstable ejection. In addition, in the case of ink using a pigment dispersed with aid of a surfactant or a dispersant such as polymer dispersant, the pigment as a coloring component is liable not to remain on the paper fiber surface of the recording medium surface at the printing on plane paper or recycled paper and therefore, a sufficiently high printing density cannot be obtained and poor coloring tends to result.
In general, a dispersant not adsorbed to the pigment surface from the beginning and dissolved in a solution, or a dispersant eliminated from the pigment in an environment afterward has a tendency to elevate the viscosity of ink and for adjusting the viscosity of ink to the range suitable for ink jet recording (a range from 1 to 10 mPa·S) the content of the pigment must be limited. As a result, a sufficiently high printing density cannot be obtained on plain paper or recycled paper and also good coloring cannot be obtained. Therefore, it is difficult to realize a high-quality printing image Furthermore, when an ink filled in the head is stored for a long period of time, the dispersion stability is deteriorated and the ink may not be easily ejected through a printer nozzle.
On the other hand, as for the ink jet recording method, a method of applying a polyvalent metal salt solution to a recording medium and then applying an ink composition containing a dye having at least one carboxyl group is disclosed in JP-A-5-202328. According to this method, for example, JP-A-6-106735 discloses that an insoluble complex is formed from a polyvalent metal ion and a dye and by virtue of the presence of this complex, a high-quality image having water resistance and free of color bleeding can be obtained. Also, a method of attaching a liquid composition comprising an organic compound having two or more cationic groups per one molecule onto a recording medium and then performing the recording with an ink containing an anionic dye is disclosed in JP-A-63-299971.
Furthermore, a technique of reducing the bleeding by bringing a pigment ink comprising a pigment dispersed with a cationic AB block polymer into contact with a pigment ink comprising a pigment dispersed with an anionic AB block polymer is disclosed in JP-A-7-145336; a method of contacting a black ink with a color ink using polymer dispersants different in the polarity for the black ink and the color ink is disclosed in JP-A-8-80665, JP-A-9-20070 and JP-A-9-25442, where generation of feathering is suppressed, blurring in the boundary between adjacent different colors is reduced and excellent coloring property can be obtained; and a method of combining a black ink using a self-dispersion type surface-treated pigment having a cationic group on the carbon black surface with a color ink containing an anionic substance such as anionic dye is disclosed in JP-A-10-183046, where a good color image free from blurring between colors can be realized.